The invention relates to press bending of heated materials, with opposing bending tools that are moved toward and away from each other. More particularly, the invention relates to a press bending station for the bending of heated glass sheets, with two bending tools that are moved toward one another.
Typically, in press bending, an essentially solid male mold, also known as a full-faced mold, forms one bending tool, while the other bending tool is designed as an annular or ring-type female mold. Also, in order to aid in the bending process, a plurality of suction holes are placed in portions of the full-face mold that are determined by the configuration of the annular mold when the annular mold comes into contact with a heated glass sheet during the press bending process.
As described, when a source of vacuum is applied to the male mold, the negative pressure created is transmitted through the mold via the suction holes thus causing the glass sheet, for example, an automotive laminate, such as, a windshield, or a glass sheet, such as, a side lite or a back lite (that may be tempered subsequent to bending), to be drawn to the molding face. As used herein, full-face mold denotes a bending mold against which the glass sheet lies over its full area during bending, and annular mold denotes one which supports the glass sheet only at the edge during bending.
The glass sheet to be bent is heated to the bending temperature and brought in a deformable state between the two bending tools. The latter are then moved toward one another, whereby the extensive full-face mold presses the glass sheet onto the annular mold. Shaping of the edge of the sheet thus takes place.
At the same time, the middle area of the sheet is held by vacuum against the molding face in order to perform further shaping. These procedures have to take place relatively quickly, since the glass sheet cools down rapidly and after a short time the edge area of the glass sheet falls below the bending temperature.
Various combinations of bending tools are conceivable. A combination that has been particularly well tried and tested is one in which the annular mold is curved in a concave manner, while the full-face mold has a convex curvature. The annular mold normally forms the lower mold half, and the upper mold half can be moved vertically from above and towards the latter. Instead of this, it is also possible to move the annular mold towards the full-face mold or to move both bending tools toward one another.
Following opening of, and removal from, the bending tools, the glass sheet should possess a desired shape, be dimensionally stable, and not be optically distorted. Otherwise, the bending process results in waste or products that possess poor quality.
Some of the factors that influence the quality of the products produced by the bending process are: a) attaining and maintaining the deformable state, b) positioning and slipping of the glass sheet in the molds, c) speed of execution, d) controlling mold and/or tool contact with the glass sheet or molded part, and e) contamination of bending product surfaces.
Specifically, heat gained or lost by the glass sheet throughout the press bending process can cause the glass sheet to be incorrectly bent, to crack, break, optically distort, and/or dimensionally change shape and size. Varying surface contours of the full-face mold can make it difficult for the mold to properly hold, position, prevent slippage of, and release the glass sheet during the press bending process.
A partial vacuum present at the mouths of the suction holes and slow execution of the bending process can produce in the glass sheet, as an unavoidable side-effect, local cooling zones which can impair the optical properties of the glass sheet at these points. In the case of the partial vacuum condition, the molding area of the full-face mold is kept free from suction holes. See EP 0 530 211 B1 which describes a full-face mold of the type mentioned at the outset.
Molds and bending tools contacting the heated glass sheet can also induce physical or optical distortion of the product. In addition, particulates originating from a variety of process or external sources can mar and distort the glass sheet surface during the bending process. Currently, the glass sheet to be bent is heated prior to the press bending operation, for example, in or just outside of a glass sheet preheating furnace. At times, the molds are heated by their own heat source, for example, electrically, with hot oil, air, or various other fluids. Even with these heating considerations, improvements in controlling the heating of the glass sheet could still be made.
Also, it is still difficult to consistently and properly position, hold, and release the glass sheet at the mold surfaces, and to prevent slippage of the glass sheet during the press bending process. Further, opposite mold surfaces place pressure on product viewing areas, which can result in the localized cooling that may lead to physical or optical distortion.
To minimize glass sheet surface distortion, due to process particulates being pressed between the mold and glass sheet surfaces, a second material, for example, a stainless steel cloth, has been included between the mold and the glass sheet surfaces.
However, the press bending process is still capable of improvement in respect of the bending speed, the bending accuracy, and the optical quality of the bent glass sheet produced with such molds. Thus those skilled in the art continued to seek a solution to the problem of how to provide a better press bending station for the bending of heated materials and particularly glass sheets.